A protest in Mexico City against alleged voting irregularities in the country’s presidential election. Photograph: Eduardo Verdugo/AP

Growing allegations of systematic vote-buying and other irregularities are casting a shadow over Enrique Peña Nieto's still unconfirmed victory in Mexico's presidential election.

The preliminary count from Sunday's poll gave Peña Nieto of the Institutional Revolutionary Party, or PRI, about 38% of the vote and a six-percentage-point lead over Andrés Manuel López Obrador of the leftwing Party of the Democratic Revolution, or PRD, who is contesting the result.

The most notorious pre-election allegation was that the PRI distributed huge numbers of cards for pre-paid purchases at the Soriana supermarket chain in return for votes. This appeared to be confirmed this week when shoppers rushed to use the cards, worth between 100 and 700 pesos (£5 to £35). Soriana has denied any wrongdoing.

Other alleged irregularities include the PRI's use of children to accompany voters into polling booths to check how they mark their ballots, reported by the Mexican organisation Civic Alliance. The group issued a report saying 28% of those interviewed by its 500 observers across the country had been exposed to some form of vote-buying or coercion. Gift-giving during electoral campaigns is permitted but making it conditional on votes is illegal.

The PRI party machinery has long been famous for using handouts and intimidation to push the limits of the law, particularly in poor communities. Other political parties are also regularly accused of doing the same in their own bastions, though on a smaller scale.

This election, however, has exposed these practices as never before thanks to the explosion of mobile phone ownership and social media, as well as the activism of the new anti-PRI student movement that erupted in May.

In another video two people from the state of Puebla say they were bussed to the state of Veracruz to vote for the PRI, supporting charges of a party operation to use up the limited number of ballots available at polling stations where vagrant voting is permitted.

Another video shows people who say they were hired to supervise polling stations for the PRI protesting about not being paid yet.

Although such charges may damage Peña Nieto's claim to represent a "new generation" of PRI politicians, few observers expect the upcoming legal challenges to prosper given the 3 million vote margin of his victory, and the high standard of proof required by the electoral courts.

Meanwhile, the PRI insists the election was fair. "Enrique Peña Nieto will be a legal and legitimate president," his campaign manager Luis Videgaray told MVS Radio on Wednesday. He nevertheless added: "Of course our democracy is perfectible."